1. Field of the Invention
The present invention relates generally to an apparatus and method of changing between a Data Over Cable Service Interface Specifications (DOCSIS) mode and a DOCSIS Set-top box Gateway (DSG) mode in a digital broadcast receiver and, more particularly, to an apparatus and method of changing a mode in a digital broadcast receiver, capable of smoothly changing between a DOCSIS mode and a DSG mode in the broadcast receiver.
2. Description of the Related Art
In general, DOCSIS, as a standard interface of a cable modem for processing input and output of data (signals) between a cable television operator and a personal or business computer or television set, is known now as “CableLabs Certified Cable Modem.” DOCSIS 1.0 was ratified by the International Telecommunication Union (ITU-TS) in March of 1998.
Cable modems conforming to DOCSIS are now being marketed. However, cable operators can support their existing customers who have non-standard cable modems by adding a backwards-compatible DOCSIS card in order to handle their existing customers. As DOCSIS continues to evolve into new versions, users of the existing modems can upgrade DOCSIS to the newer versions by changing the program of the Electrically Eraseable Programmable Read-only Memory (EEPROM) in the cable modem. The cable modems supporting DOCSIS show a tendency to be integrated into set-top boxes for use with television sets, and must also support high definition television. The set-top box itself follows a standard known as OpenCable.
DSG is a transmission standard for supporting digital television in the OpenCable standard. DSG is a digital cable television data transmission standard for performing bi-directional communication, which includes performing bi-directional communication for transmission and reception with a service information server, transmission and reception with a conditional access server, and recovery function of uni-directional mode, upstream channel and downstream channel in addition to performing as a data modem through the existing DOCSIS.
Digital broadcasting includes digital terrestrial broadcasting, digital satellite broadcasting, and cable digital broadcasting according to a transmission medium. The current digital satellite broadcasting has be activated all over the world, and is being tested in the United State of America and some countries in Europe.
Digital broadcasting very efficiently compresses an amount of information of channel-specific programs on the basis of a Moving Picture Experts Group (MPEG) coding standard, and very efficiently multiplexes and transmits programs corresponding to a plurality of channels on the basis of transmission medium-specific digital modulation schemes. Thus, digital broadcasting can broadcast several tens to hundreds of channels without using a great deal of repeaters as in analog broadcasting.
A broadcast system for cable digital broadcasting is generally composed of a head end for transmitting a broadcast-related digital signal and receiving and processing data uploaded from a subscriber, and a set-top box for receiving a digital signal transmitted from the head end and converting the received digital signal into an analog signal to thereby reconstitute it into its original audio and video signal.
The head end refers to a main control center equipped with technical equipment capable of a cable television system receiving, creating and re-transmitting a program from a satellite or another place via a system network. The set-top box refers generally to a domestic communication terminal required for multimedia communication services inclusive of video-on-demand services.
With this configuration of the broadcast system, each System Operator (SO) creates programs, contents and other data of the cable television, or is supplied with them from creators to then transmit them to subscribers. The subscribers can watch their desired broadcast after looking at the programs or the other data transmitted from the SO.
Video and audio compression technology for use in the cable digital broadcasting employs MPEG-2 as a standard in the world. Its modulation scheme employs one digital modulation scheme, namely a Quadrature Amplitude Modulation (QAM) scheme capable of changing an amplitude and a phase according to a digital data signal to be transmitted, which is suitable for higher-efficiency transmission of data within a limited transmission band. In particular, 64-state QAM digital frequency modulation technology is mainly used to transmit downstream data on a coaxial cable network, and supports a maximum data rate of 28 Mbps on a single 6-MHz channel.
Most technologies used in cable broadcasting are used for digital television. One difference between them is that digital television technology makes use of a wireless transmission medium while cable television broadcasting makes use of a wired transmission medium. Thus, cable broadcasting has an advantage in that it is better than the terrestrial broadcasting in bi-directional services.
The digital broadcast receiver, such as a set-top box, for digital broadcasting is a device which is mainly directed to deciphering digitalized broadcasts and management information transmitted from the head end, which is installed at a local cable television firm, for the cable television or modem services provided to the subscribers and transmitting analog signals to television receivers. The digital broadcast receiver receives the broadcast and management information from the head end, such as an audio-video server, a service information server for transmitting and receiving broadcast channel information, or a conditional access server.
The cable digital broadcast receiver for cable digital broadcasting based on the cable network includes a cable modem in many cases. The cable modem is a device for converting an analog signal into a digital signal so as to make high-speed Internet service available through a cable network. The name “modem” is proper because the cable network is an analog network like a telephone network. The telephone network is made up of copper wires, and the cable network is made up of coaxial cables or optical cables. Accordingly, the cable network has a much wider bandwidth than the telephone network. However, the cable network requires modulation and demodulation technology for converting digital into analog, and vice versa when transmitting data. It is the cable modem that is being developed to support this.
The cable digital broadcast receiver including the cable modem in cable digital broadcasting is basically composed of a plurality of broadcast channels and a single modem channel. The broadcast channel and the modem channel are distinguished by a physical frequency, and cannot overlap each other, so that they have a reciprocal exclusive relationship. Information on the broadcast channels is managed by Service Information (SI), while information on the modem channel is adapted to find its own channel via a self search process.
A cable digital broadcast system generally has a structure where a local cable broadcast station and a subscriber side are generally connected through a cable network. Subscriber-side equipment includes a cable digital broadcast receiver having a cable modem, a television (TV) receiver, etc., and local cable broadcast-side equipment includes an SI server for transmitting broadcast channel information, a CA (Conditional Access) server, a Cable Modem Termination System (CMTS), an audio-video server, a broadcast head end, etc.
Data transmitted from the local cable broadcast station to the subscriber-side equipment includes additional information such as broadcast program information except video and audio signals, and are transmitted in packets. The cable digital broadcast receiver receiving this information processes broadcast signals received by the TV receiver and by the cable broadcast station, and converts them into analog signals capable of being recognized by the TV receiver.
Hereinafter, the equipment located at the local cable broadcast station will be described first.
Devices associated with Internet signals are the CMTS, the CA server, and the SI server. These devices are interconnected via an Internet Protocol (IP) network.
The CMTS is a cable modem head end, which is device for converting data of a cable modem into an Internet data packet. The CMTS provides some functions including routing for storing local data in the cable system, filtering for protecting a cable operator from undesired hacking and traffic embodying for guaranteeing a Quality of Service (QoS) to the subscriber.
The SI server transmits channel information of the cable broadcast, program map information, event information to the subscriber side, and the CA server permits access to a specific service or content only to an authenticated user.
The devices associated with the broadcast signals include the audio-video server and the broadcast head end.
The audio-video server compresses and digitizes MPEG audio and video signals and transmits them to a subscriber-side MPEG(Host) via the broadcast head end.
The broadcast head end refers to a main control center having technical equipment for receiving, creating and re-transmitting programs from the audio-video server via the network.
The local broadcast station side constituting the abovementioned devices communicates with the cable digital broadcast receiver, adopting the CMTS and the broadcast head end as terminal points for connecting to the network.
In the case of communication between the cable digital broadcast receiver and the subscriber side, there is a signal stream from the CMTS to the subscriber side, and vice versa. In other words, communication between the cable digital broadcast receiver and the subscriber side is bi-directional.
In contrast, in the broadcast head end, there is a signal stream from the broadcast head end to the subscriber side, but not in reverse. The broadcast head end is characterized by uni-directional communication.
Equipment required to watch the cable digital broadcast on the premise includes the cable digital broadcast receiver.
The cable digital broadcast receiver is essentially composed of the cable modem, the MPEG(Host) and so forth.
The cable modem makes it possible to connect a Personal Computer (PC) to a cable TV line and receive data at a maximum data rate of 10 Mbps or so. When the cable modem is installed together with the set-top box, it is possible to watch the television. All of the cable modems should be connected to a coaxial cable of the cable television firm in to communicate with the CMTS of each local cable broadcast station. Each cable modem can transmit and receive data with respect to only the CMTS, but it cannot exchange signals with the other cable modems connected to the same line. The real bandwidth of the cable TV line for Internet services is a maximum of 27 Mbps or so downstream (from the broadcast station to the subscriber side) and 2.5 Mbps or so upstream (from subscriber side to the broadcast station).
The cable modem function of the cable digital broadcast receiver enables the Internet services of the PC of a user by connection to the user PC, and is used as a return path of a conditional access device which the cable digital broadcast receiver includes, thereby being used as a path for communicating with the CA server. When the cable digital broadcast receiver has a built-in data broadcast application, it is used as the communication path of bi-directional data broadcasting.
A standard for the cable modem is originally made to utilize a cable television network, which has been already established, to provide the Internet services. The cable network installed by each communication company supports a maximum bandwidth of 750 MHz, wherein an upstream channel and a downstream channel are used one by one. Furthermore, a frequency band uses 6 MHz per channel.
Among them, the upstream channel mainly uses a Quadrature Phase Shift Keying (QPSK) scheme, while the downstream channel mainly uses a Quadrature Amplitude Modulation (QAM) scheme. The downstream channel uses about a maximum of 30 Mbps with a 64 QAM scheme that is widely used at present. The upstream channel uses 10 Mbps when using all of the 6 MHz bandwidths. However, the upstream channel does not use all of the 6 MHz bandwidths, but rather uses about 2.4 Mbps due to noise or other various influences. Because of this noise, the upstream channel is slower than the downstream channel. In order to protect the network equipment from the noise generated by devices installed at each home, the QPSK scheme, which is capable of reducing the noise a bit more, is used. By contrast, the downstream channel uses the QAM scheme because speed is important.
The MPEG (Host) demodulates video and audio data from the received MPEG data to enable a video and audio processor to process the demodulated data, wherein broadcast program information as additional information from the MPEG data is demodulated and transmitted to a Central Processing Unit (CPU).
The overall operation of the cable modem is as follows.
First, a frequency of a channel used for downloading is scanned. When being turned on, the cable modem automatically finds the frequency used for uploading and downloading. The frequency is different according to a cable modem operator. When the frequency is found, it is locked to the corresponding channel.
After searching and locking the frequency, the cable modem receives data containing contents of the channel used for uploading, or an Upstream Channel Descriptor (UCD), and communicates with an external network using this information. When the cable modem fails to receive the UCD, it makes an attempt at upstream channel recovery after setting a DSG unidirectional mode.
The cable modem then checks the intensity (referred to as a “level”) of a signal being transmitted on uploading. When setting of the channels used for uploading and downloading is completely terminated, the cable modem provides access to a Dynamic Host Configuration Protocol (DHCP) server and is assigned an IP address and a gateway address. The cable modem reserves the assigned addresses and then transmits them to a LAN card of the PC.
A server supplies the cable modem assigned the IP address with a ‘configuration file’ which contains various information required for operation other than frequency information. The cable modem uses a protocol known as Trivial File Transfer Protocol (TFTP). The configuration file is different for each cable modem. Cable modem operators can restrict uploading and downloading speeds by using the configuration file.
The DHCP server leases the IP address to the cable modem on the basis of a preset time period (e.g., 24 hours, 12 hours, 1 hour, etc.). When the cable modem is still being used after the preset time has elapsed, the cable modem is allowed to continue to use the leased IP address. In order to calculate this time, the cable modem is supplied with a current time from a server, wherein the server is the DHCP server for the most part. When the IP address is not assigned from the DHCP server, the cable modem makes an attempt at a DSG uni-directional mode.
With a series of processes mentioned above, an environment where users can normally use Internet is set.
In an OpenCable digital broadcast system, the digital broadcast receiver operates in a DOCSIS mode as soon as the system begins to operate.
Then, when an instruction of a DSG mode is issued from a user host, the digital broadcast receiver attempts to operate in the DSG mode.
In other words, a process of searching the channel is performed for digital broadcasting. When all the channel searching processes fail to secure the channel, the digital broadcast receiver runs to the DOCSIS mode after searching the channel.
In the OpenCable digital broadcast system, the digital broadcast receiver is adapted to change the operational mode between the DOCSIS mode and the DSG mode. However, only the minimum requirements are prescribed in the OpenCable digital broadcast system, so that it is impossible to cope with exceptional cases which are generated under real various network environments and broadcast systems.